There are several considerations in terms of migration from on-premise exchange 2003 to Office 365. There is no definitive guide since there are so many ways you can approach the migration and deployment.

The things you have to consider are

Current environment

What features of exchange are your users using right now that is a must in the cloud as well (public folders ...etc)

What special considerations do you have on the on-premise environment (applications that integrate with mail, relaying to external contacts, ...etc)

These are the main discussion points, once you know the path you are willing to take, this is when you draw plans either by own IT staff or through authorized Microsoft Partners such as 'Cloud Consultancy'